Why some fish are really bad at swimming

“Fish gotta swim, birds gotta fly,” Jerome Kern wrote in a Broadway love song—but while it’s sweet as a lyric, it’s not entirely true.

Not only are there plenty of awesome birds that can’t fly, like ostriches and penguins, but a number of fish don’t swim as well as you’d think. That made us wonder: Why is it that some fish wouldn’t medal in swimming?

It turns out they all have their reasons for investing their energy elsewhere.

Slow But Sure

When it comes to fish, “slow swimmers tend to be those that don’t need to swim quickly or efficiently,” says Selina Heppell, head of the department of fisheries and wildlife at Oregon State University. This includes ambush predators that get the jump on prey without knocking themselves out.

Seahorses’ unique body shape makes them some of the slowest fish in the sea, for instance, but they’re incredibly fast predators as long as the waters around them are still. They eat copepods, tiny crustaceans that can flee in as little as two milliseconds. But even that’s not fast enough: A seahorse can strike in one millisecond. (Related: Why does the seahorse have its odd head? Mystery solved.)

Other fish are masters of blending in. Stonefish perfectly camouflage themselves on the seafloor and sometimes even grow algae on their bodies. This perfects their camouflage and may also help lure fish to their demise.

When a fish wanders by looking for a meal of algae, a stonefish snaps it up in its huge jaws, often swallowing its prey whole. This also protects the algae from getting eaten, since the fish gets eaten instead.

And since they’re covered in spines so venomous they can kill a human in under an hour, stonefish have no need for speed.

Other fish on the slow-food diet are deep-sea predators that “hover in the dark waiting for prey and don’t have much muscle mass,” Heppell says.

Blob sculpins, also called blobfish or fathead sculpins, live as deep as 4,000 feet (1,200 meters) off the Australian coast. These fish don’t have any muscle mass, making them less dense than the deep water they live in and therefore able to survive crushing deep-sea pressures. Their other trick is lying still and eating whatever crabs, mollusks, and other animals that happen by on the seafloor.

Will Walk for Food

Flying gurnards, a bottom-dwelling fish, can swim but certainly won’t set any speed records, says Scott Heppell, associate professor of fisheries at Oregon State University. Gurnards are a type of sea robin, fish known for their wing-like pectoral fins are used to attract females.

They’re one of a very few fish that choose to walk on the seafloor instead of swimming, because speed just isn’t that important to them. Rather than chasing down prey, they slowly poke around for it.

Their pelvic fins aren't webbed at the ends, so they can dig into the sand to find snacks with fin rays that work like fingers. (Related: Why this fish “walks” along the ocean floor.)

Another sea walker is the spotted handfish of Australia and Tasmania, which almost went extinct in the mid-1990s. Pollution and egg predation are among the causes of the handfish’s plight, but Australian scientists have begun a breeding program to give them a helping hand, so to speak.

But the weirdest-looking “walking” fish of all is probably the red-lipped batfish, which strolls across the undersea world of the Galapagos.

One last fish that doesn’t care for life in the fast lane is the pufferfish, also called blowfish. If chased, it just swallows a lot of water and inflates to be so large, and in some cases so spiky, that it can’t be eaten.

Pufferfish are also toxic enough to kill 30 humans, with no known antidote.

So be careful that they never hear you call them bad swimmers.

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